Accurate knowledge of the amount of film remaining in a film canister is important for camera systems used for computer output microfilm. These camera systems are typically connected either to a host computer, a magnetic tape drive, or some other equipment which has stored blocks of data which are to be printed on the film by the camera system. These data blocks vary in size and anywhere from a few feet to several hundred feet of film may be required to print the data. It is important to know the length of film remaining in the canister before the printing of a block of data is started so that there is enough film in the canister to print the complete block. This will allow the user to load a new full roll of film rather than have to splice the film in the middle of a data block.
Another reason it is important to accurately know the length of film left is because some applications require that a substantial length of film be left unprinted at the very end of the film to facilitate threading into developer equipment. The accurate knowledge of length of film left allows the camera system to automatically stop when a predetermined amount of film is left and therefore prevent the loss of data due to exposure to light during the threading process.
Determining the amount of film in a film canister has either been inaccurate or inconvenient with prior art devices. Two methods have been used in the past. One device that visually indicates on the side of the canister the amount of film left in the canister incorporates a lever mechanism which contacts the outside of the film roll. This provides only a relative reading with poor accuracy. The operator must stop the camera system and open the film bay area to read the amount of film left. This causes waste by exposing unprocessed film.
A second method is a meter-only system which allows for the display of film left information on an external device such as a CRT screen. It uses metered feed rollers to determine the amount of film removed from a canister having a predetermined starting length of film. This system simply subtracts the amount of film metered out from the known starting length. This method, due to accumulating metering errors, provides relatively poor accuracy as the canister approaches empty. The accuracy of this method also can be seriously degraded by "soft" errors of the system (hardware or software) which lose blocks of metering data. Additionally, this method is inconvenient because canisters are sometimes removed before the film in them is used up. This requires that the amount of film left in a partially used canister, as determined by the metering system, be written on the canister. The recorded length of film remaining in the canister must be manually entered into the system when that canister is inserted or reinserted.